Finder.



L BECKER.

FINDER.

APPLICATION men Nov.23. 191s.

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Patented D60. 26, 1916.

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J. BECKER.

FINDER.

APPLICATION FILED NOVIZB. I9I. 1,210,137. Patented Dec. 26,1916.

3 SHEETS-SHEET 2.

In a. Plano-Concave Lens sc' S"K= o Jenaheaviest flint, 49 S 57. Refractive Index )Ln |-96z6 J. BECKER.

. FINDER.

APPLICATION FILED NOV.23, 1916. 1,210,137. 'Patented Dee. 26,1916.

3 SHEETS-SHEET 3.

.HOW

JOSEPH BCKEB, 0F WASHINGTON, DISTRICT OF COLUMBIA.

irrational..V

s'pecineatib of Letters latent. Patentedibec. 26, 1916. i

Original application led February 17, 1905, Serial No. 246,169. Divided and application led October 10, 1916, Serial No.-124,8 32. Divided and this application illed November 23, 1916. Serial No. 133,093.

To all rwhom it may concern.'

Be it known that I, JOSEPH BECKER, a citizen of the United States, residing at lVashington, in the 4District of Columbia,A have invented a new and useful Improvenient in Finders, of which the following is a specification.

The present application,

continuation of my prior and herein merged -application .Case An, which' was filed October 10, 1916, as Division one of my Case J, Serial No. 246,169, filed February 17, 1905, and it relates to certain features of said Case J which are separately patentable under the head of finders, `such patentable features being placed on"a more scientific and precise foundation in the present application, Case Ao, by the introduction of what I call the co-center K.

My present'invention consists in improvements on the eccentric lens finder disclosed in my prior application, Case Ah, Serial No. 87 ,987, filed March 31, 1916, as a division of. my Case C now Patent 1,178,476; and it is incidentally "related, by way of its removable lens element of `Figure 8, to the invention shown in my Case AL, Serial No. 110,310, filed July 30, 1916, as a division of my case F now Patent 1,198,947. l

A simple eccentric divergent lens, as compared with the most nearly corresponding centric divergent lens, necessarily comprises much thicker parts whose refracting face elements are set at a considerable inclination to each other, and are, therefore, apt to produce, in the finder rays, excessive refractions, and as a result,A in the finder image,`effects of barrel distortion which d0 not necessarily interfere with the operation and use of the finder, but which are nevertheless objectionable.

The barrel distortion produced in the equi-concave type of eccentric lens, for instance, is so great, that I have always preferably used plano-concave lenses instead. placing them as shown in Fig. 8 ofmysaid Case C with the plane face turned toward the object. Distortion is further reduced, I

i. identified for' convenience of reference as CaseAo, is a have discovered, inl a very marked degree by tilting the thicker edge of the lens toward the object so that the principal axis ofthe lens, instead of being horizontal as indicated in the said Fig. 8 of my said Case 0, shall be inclined to pass close by, or even into the observers eye; and this last feature constitutes the gist ofmy present invention.

.The principle involved in inclining the single eccentric lens is also applicable to compound eccentric lenses; but in these, there is a special advantage in applying the principle to each element separately, and as a result the principal axes of the'two com-` ponent lenses can not be alined or even parallel; but the main or ycommon axis of the combination is inclined, as in the single eccentric lens.

Incidentally my invention consists in a' specially convenient form of mount for the rear or eye lens of the combination to facilitate its removal from the position of use, either for long focus -work or for cleaning.

In the accompanying drawings :--Fig. 1' is a vertical section of a single half lens form of my improved finder of the type having a reflected bead sight 108 as in my said Cases,

lenses L and L are conveniently cut out of a single full centric lens.' Fig. 3, with related equations annexed thereto as a part thereof, is a diagram illustrating and serving` to prove the geometrical properties yof what I call the co-center K, which has the remarkable optical property of being the focus of all in-glass rays whose air elements suffer minimum deviation. Fig. 4, with a related equation annexed thereto as a part thereof, is a diagram showing a full centric lens whose two face centers R and S and cocenter K, are arranged and spaced in the same .manner and at the same inclination as the three corresponding centers R', S', and K of the half lens L in Fig. 1; and, therefore, this full lens of Fig. 4 comprises an eccentricsegment C-4-5 which has the same shape, size, inclination, and refractive pipperties as the half lens L used in 1. 1g. 5 is a diagram of an equi-concave ens whose bring its co-centerK from right infinity in continued untilthe first or toward the lens, the bending process being lens has become flat, and `showin how the two minimum rays U-U and slued'around to meet at the right in the real focal point K, which is the image of the cocenter K in its newposition. Fig. 7 is a cor-4 rect drawing showing how the eccentric lens having minlmum deviation, is desi ed to yield a given angular field lZAZ o 90 degrees for a given position E, of the observers eye; all angles and dimensions being correctly drawnbut made specially lar e and clear by using a very thick lens and se ecting an optical (glass of the highest available refractive in ex, nearl equal to 2. Fig. 8 is a vertical section ofY a compound form of finder, with the rear lens hinged for quick removal, such arrangement beinguseful, not only to facilitate cleaning of the inside lens faces, but principally to permit of quickly and convenlently increasing the focal length of the finder when the coperating camera objective is also compound and comprises an elementthat is removable-for long focus work.

Improvements to secure a more perfect image- These improvements relate mainly to t e eccentric lens and consist principally in so building and setting it in its mount or on the camera that its principal axis, instead of being parallel with the principal axis of the camera objective, as in my said prior Case C, and Case Ah division of C, shall lie in ,an inclined position thereto, so as to pass close by or even into the operators eye. When the finder lens is composed of but a single lens, this condition 1s satisfied by properly inclining the lens, to tilt its thicker edge forward, that is, in a direction away from the camera plate or film or away from the operator and toward the ob'ect. Re-

ferring tothe diagram, Fig. 8 o my said prior Case C, this tilting may be supposed to take lace about the thinnest point of the lens so t at it shall have no tendency to displace the central point of ima e f. The only effect and object of nsuch-tilting is then to reduce distortions in the image f as explained fully hereinafter, and, therefore, the

vgeneral principles predicated of such lens .and image in my'said prior Case C apply .V: of Fig. 5 have, during the bending process, been 'when the without any vfurther qualiationtoY the tiltcessive curving o the rear `face. This lens is tilted as indicated b the inclination 'of its principalaxis RCS, u

The inclination shown' 's that in which the image formed by the lensis the moet nearly perfect. .'That is, if the lens lwere tilted either farther or les than shown deformation would be introduced in the finder dueto variations in refraction. In

through the thicker parts of the lens in t eir 'course throu h the lensmeet the lens faces at equal ang es, aid suil'er the least refraction; and from theoretical demonstrations which Iv give later on, it will appear that a divergent lens which is at all suitable as a finder lens, will, when mounted as here specified, usually send all or nearly all of its 'minimum rays into the observers eye.

ject and with its deeper concave face toward the observer, The point of view should be enough higher than the lens to center the), ima e Yof the object in the lens. Then tilt the ens back and forth, noting that the vertical dimension of the image lvaries, and

that in tilting from one extreme position to.4

the o posite extreme position the said vertical dimension of the image will vfirst increase to a certain maximum and then decrease. It is in an intermediate position where the image is tallest that the im has the least image;S the tj position the rays of light that ass distortion and that the lens 1s at the proper inclination.

As a rulelenses that are planoconcave, or nearly so, will be properly enough inclined neral dlrection of the concave or eye face o the lens is vertical, that is, when the lens is mounted in the camera with the general direction of its deeper concave face parallel with the camera plate.

My invention involves certain geometrical relations which are peculiar to it and which I shall now set forth.

The extreme `finder rays 120 and 121 produced beyond the lens L intersectin a point A at an angle lw which is equal to the corl `responding angular value of the camera field. The refracted parts 120 and 121 of these same extreme rays proceed to meet 1n the operators eye at E.' Points E and A are, therefore, conjugate foci of the lens and must be in alinement with C which represents the thinnest point `of the lens as well as its two, in this case, nearly coincident nodal points. Moreover, as by the present invention the principal axis RCS ofthe lens passes close by or even into the eye. E the focal length fof the lens can be derived directly from the standard lens formula:

f CA CE p It should be noted that A is not a purely imaginary point, for any person being photographed sees, in A, a reduced image of the loperators eye E, and the extent of field seen by the eye E through lens L is exactly the same as wouldbe seen if the lens L Were removed and the eye E were shifted to A. I, therefore, call A the virtual eye or the virtual position of the operators eye.

Point R is the center of curvature of the first refracting face of the lens L, and point S is the center of curvature of its second refracting face; so that RS is the principal axis of a full lens, such as lens L2L3, Fig. 4, in which the present lens L was comprised as eccentric segment C 4-5. Point K, also on the principal axis RS is what I call the .co-center K of the lens. We shall presently f most convenient, here SQ, must be reversed. The reversal is made plain in Fig. 3, where the process followed for finding the old optical center rC is illustrated in conjunction with that to be followed in finding my co-center K.

Fig. 3, for clearness, shows a lens of considerable thickness R to S, but drawn to scale and in which the radii RR and SS are both positive, so that the equations which are annexed to the figure asa part thereof, may easily be verified by computation s well as by scale.

The `optical center C, Fig. 3, may be defined as the geometrical point C of the lens axis RS, through which is directed any inglass ray, such as -ray PQ, which connects parallel `surface elements of -the lens,'these elements being indicated by heavy shading at P and Q.

My co-center K,-Fig. 3, may similarly be defined as the geometrical point K of the lens axis RS through which is directed any 1nglass ray, such as ray PP-, which'connects surface elements P and P of the lens that are anti-parallel with reference to such inglass ray PP. 1

The ray PP is the minimum ray of the prismatic lens element which it traverses, for it meets the two refracting faces of the lens at angles that are equal but oppositely directed. The equality of these loppositely directed angles at P and P is proved, in Fig. 3, by observing that TP and SQ. being parallel by construction, must determine with the two straight lines crossed in P', two similar triangles TPP and SPQ. The lattertriangle SPQ,whose sides SQ and SP are invariable radii, is necessarily isosceles, and therefore TP and TP', which are the normals to the surface elements at P and P, are equal in length and equally inclined on the ray PP. This proposition is true for any possible inclination of the ray PPK. Thus, in Fig. 4, noting that the in-glass rays 2 3, 4 5, 6 7, 8 9, all converge in the co-center K, we can be sure that each is vthe in-glass elementof a ray whose two air elements suii'er minimum -relative deviation; and, we can be equally sure, that an in-glass ray, such as 8 10 whlch is directed to miss the co-center K, is the in-glass part of a ray whose two air elements suffer a relative deviation that is greater than the minimum.

My co-center K and the optical center C are alike in that their distances to the centers of curvature R and S are proportional to the radii of curvature r and s; but they differ in that one of the radii used for determining the co-center K must be an imaginary or reversed radius, such as the radius SQ used in Fig. 3, and in Fig. 4.

In Fig. 3 it will be noted that the distances CS, CR are both'positive, whereas the distances KS and KR differ in sign, for KS is negative 'and- KR is positive. The difference between the two centers Cv and K may accordingly be expressed algebraically as follows:

CR r

That is to say my co-center K is really the optical center of a lens whose second face would be at S instead of at S, and whose thickness would be RS.

The positions of centers C and K are more conveniently determined by means of tem of Optics, Cambridge 1829, pages 88 vand 89 (copy at the Library of Congress).

r't y i Rfk-m (3) '2m-Ht, RK: r+s (4) y My symbols 13-3 and t, are the same as those used by Coddington except that his convention with reference to signs 1s differ-v ent ;d but the final result is the same.

The divergent lenses used as finder lenses are comparatively thin, and may be considered as -lenses having their two nodal points, as well as the three points R, S and G, of Fig. 3, all merged in the single point C of Fig. 4, for, if in equation 3, we make t equal to zero, RC also becomes zero. But if we make t equal zero, in equation 4, we obtain 2 r' RK.-. Tlf, (e

or, in a thin lens, Fig. 4, I 2rs CKa-o) (6) From this we easily derive v .esce e which is a very important equation, for the three reciprocals 1 l CK which is equal to the average curvature of the two lens faces.

If the lens of Fig. 4 could be made of a material having a very high refractive index, say as high as ten, it would, lfor the same focal length, be of ve nearly uniform thickness and its generaliI sha would be a ver close approximation to t is circular :arc ICM, whose center is in K.

The curve "MCMQ Fig. 4, therefore, is

- a very exact physical representation of what is frequently referred to in optics as the shape of the lens; and as it divides the lens into two lenses L2, L3, which are equal wer, it may be called the bisecting inace of the lens.

The bisectn interface MM of an uiconcave lens, ig. 5,is plane, whereas t at in te l -in the focal length are general of a plano-concave lens Fig. 6, is curved;

' Aand the-curved'lens of Fig. 6 ma be considered as having been formed by Vnding the plane lens of Fig.k 5.

The principal effect produced inl bending a lens is to shift its co-center K, for the optical'center C, (Figs. 5- and 6) remains almost stationary, and the changes produced y negligible. The best extant finders ofthe centric divergent lens type are curved lenses and they must be used with their convexitfy turned toward the object.- The reasonL or this appears clearly on noting what takes v place 1n the plano-lens of Fig. `6, whichwe 80 must henceforth consider asa curved lens with its convexity toward the object. Here, the eye E, by simply moving up into coincidence with K willreceive each and ever one of the minimum ra s that passes throu dthe lens; and when t e eye 1s not exact y at E, it receives rays that have not suffered much more than minimum deviation.

With the equi-concave or plane lens, Fi 5,` which we might for convenience re er to as the fiat lens the eye cannot possibly receive more than one of the minimum rays at a time, for the finally emitted parts of all minimum rays diver from a virtual focus K situated beyond t e lens.

The plano-concave or curved lens of Fig. 6, if turned to face the other way, would be worse than the flat lens, but it should be noted that it is really used in what is known as its reversed position, because its spherical aberration, for points in the image, is several times larger in such reversed or lane-concave position of Fig.6, than it wo d be if turned around'.

From equations given in lines 8 and 13, page 149, Article 125, of Heath, A. Treatise on Geometrcal Optics, Cambridge, 1895, we easily derive for the relative value of the two focal aberrations:

`Aberr. plano-concave 8 Aberr. concave-plane p'2p+2 v If therefore we assume that the refrac tive index of the plano-concave lens in Fig. 6 is mu equal one and a half (p.=1.5), then according to this equation 5 its spheric al aberration for the central point of the image is twenty-seven sevenths (27/7) or nearly four times greater than it would be if the lens were reversed to act as a fconcavo-plane `lens; and it would be exactly four times ater if the refractive index were as hig as 2. (In confirmation, see table of aberrations on age 118 of Czapski, Grundzge der Theorie r Optchen I mtrmente 4'nach Abbe, Leipzig, 1904.) Spherical aberration in the image, however, is practically of littlel consequence because a divergent lens finder is virtually provided wit an entrance pupil or stopy opening what which4 is of pin-hole site, for such' stop' is nothinIg else but the diminutive pupilv of fore, is the only defect that need receive any` attention, and this is reduced in a very ap' Y preciable degree by the use lof ,.mi'nimu1.`x v rays.. f

It is alwaysr possible to design aii'inder lens so tlat eachl andV every ray received 'by the eye .shall bea minimum ray. Let it' be required, for instance, to-design the minimum-ray nderlens', which will-show to an 'eye at E, Fig. 7, a field ofninetyy egrees,

throughthe window 12. Through t e top and bottom edges' of the window 12 draw the'limiting 'raysZ and Z of the finder field,

`and v'produce them until they meet in A,

whichV is the position of the virtual eye.

DrawjAE as principal axis ofthe lens to be, and DE as vthe desired position of the highest ray from the finder to the eye.A Plot Y normal PT at P. l

The triangle PXY, I call the refractive triangle because its angles at Y and X are respectively equal to the angles of incidence and refraction at P; for, in this triangle, we evidently have.

sin. Y P X= (9 XvPY D e A second refractive triangle PXY, in every way similar to the former PXY, determines the direction PT of the normal at P. Producing thel normals TP and TP we locate the two centers of curvature R and S. on the principal axis AE of the lens, and the two lens` faces can now be drawn by taking RP'for the radius of the first face R ofthe lens, and SP for the radius of the second face S. If no error was committed, S'Q drawn parallel to PR should meet the line PK. and the circle SS in the same point Q. This lens RS, there'- lfore, has its co-center The image of K in the first lens'face R coincides with the virtual eye at A, and its image in the secondV lens face S coincides with the real eye at E. Any ray received at'E must, therefore, be a minimum ray, for any possible spherical vaberration at focus A does not appreciably have called the .virtual eye at lin Fig. 1. Distortion inthe image, therecave to the same. radius as the face 100 of `the lens. This is in 'order that the reflected rays shall traverse tw'curvedsurfaces simifor, when both of these factors are iow,

voccurs'in currentl practice, lthe same graph 101whichhas itsl cemented face made con- Vex toA fit face 100. y'The under face 102 of the prism is also curved, being made .con-

lar to those traversed -by the direct rays,

v and Ishall, therefore, produce 1a -re'ected .image 1I on the same'scale as lthe directly formed image f. The reflected raysv also 80 -traverse the cemented curved surfaces, but

they pass through them without being 4re. fracted as i@ the/prism l and lens weremade of one piece of glass. i

The lens L being inclined to secure the best image, the prism supported thereby must bek ground .so that its Areflecting face shall likewise be properly inclined. The certain latitude allowed in the angular 'set` of the lens allows ofA a corresponding latitude in the accuracy with which the prism-is tobe ground. The forward tilt of the lens f d' holds the prism clear of the lower edge of. the lens and,`therefore, the prism needl not have its upper edge ground down as indicated at 40, Fig. 10 of the said prior Case C. The side faces of the prism, to be invisi ble to the observer and leave a sharp vcut separation between the apparentl partsy of 100 the two images f and i, should be directed to point A, which is yas before noted the virtual position of the observer-s eye. Face 102 might also be exactly directed on A with the same object. In the figure the' general 105 prism 1,01, is made clear by considering its action on the rays which proceed upwardly from the sight wire 108. It rs't" forms, by refraction, the image 1 08, then, by 'reflection, the image 108" which serves as bead sight for theiinder. d

The mirror 107 is essentially a focuser element, but its presence is necessary here to complete that. part of the fnder'image which, without mirror. 107, would appear black in the peep mirror 101, and its main lpurpose is tofsupply, in ,sok completing the image, a lighted background for the black sight wire 108 whose image 108', formed by the concave surface 102, as'just explained, serves as original for the final image sight 108".

Image 108" in coperation with the peep mirror determines the height of the observers eye; but the position of the eye in azimuth is determined by the peep mirror and the bead 25 on the camera box.

- In the present form of ecntricnder, 130

Fig. 1, the lens L does not, as in some other forms, have to be of any `certain degree of eccentricity, because' the direction of the finder field is determined by sights (peep 101 and image 108) which are'both located in the obiect space of lens L where they act exclusive y on the incidental finder rays. Any change made in the eccentricity of the lens L, therefore, simply raises or lowers" the height of the eye E, and /such hei ht is immaterial, iii-the present form of lin er, as

long, as, it is sufficient to insure that the lowest utilizable ray 121 emitted by the finder is not obstructed by the edge of the camera box, near bead 25.' The lens L shown in Fig. 1 is an exact half lens because I preferably use half lenses, as the are easily made by simply splitting full enses diametrically, as shown in Fig. 2. The half lens is, in fact, a very practical anduseful form of my eccentric lens; but its main advantage resides in a feature which can only be utilizedwhen complementary half lenses are' used together asin my' Case Ap, Serial No. 133,162, filed Nov. 24 1916; and, therea5 much less distortion in the image. Considerable latitude, therefore, is allowable in the designing and mounting ofthe elements of a compound finder lens or of the compound Aes 1finder as a whole.` However, lthe principles given above to reduce distortion in single lens finders can be applied with advantage to reduce aberrations' 1n the compound lens 'fnders; especially is thisfso when the front l element of the compound finder is to be used singly for long focus work, as in Fi 11 of my said Case C. Theprinciple is applied to the front lens used singly, as ex.-

vplained in connection'withpresent Fig. 1,

and then it isalso taken into account in adding the other lens or The result of such processleads to a form of type shown in present Fig. 8, which also contains Vother novel features. The' parts that are omitted in Fig..8 are substantially the same as seen in Fig. 1, except that the bed of the camera is usually extended to supporta second cam for long focus work as set forth in Fig. nef my Said Case C.

The finder frame 500 has a cross piece 501 which forms a. box-like frame in which the front lens Lis firmly fixed by means of cement or clamps. Point C", represents the two, in this case, nearly coincident nodal points 'of' the Afull centric lens which comprises L as eccentric segment, and it also Afracted and real y proceed toward a marks the thinnest point of such centric lens. The up r rear part502 of the frame 500 is hinged)e to the frame roper at 540 so that it may be turned own into the dotted.v position. This hinged' fart, has firmly xed therein the rear ens Point C .represents the two, in this case, nearly .coincident nodal points of the full centric lens which comprises lens L as eccentric segnient, and it also marks the thinnest point-of such centric lens. In Fig. 3 of my said prior Case C, the rear lens is mounted so it may easily be unmounted and removed for long focus work; the rear lens L here shown is removed, without being unmounted,

simply by being turned down into a position where it is out of the way; and, when turned up, it is held closed by a spring catch 515.

The from lens L isinclined according to the principles of Fig. v1. Supposing the rear lens to be in its turned down or dotted position, the extreme finder rays 510, 511,

limiting a field represented by angle v when produced without refraction, meet in A",

which is the virtual positionl of the operators eye for long focus work. v-

For short focus work the, extreme finder rays limit a -eld 2v which may have any" value required by the camera, but is here supposed to have just twice the value of v, because this is the usual pro rtion between the short and long focus fiel of the camera.

These extreme rays 510', 511', rodhced without refraction, meet in A', which is the 10c virtual position of the observers e e for short focus work. But the rays 510 ,511', v in passing'throu the first lens L, are reoint A7 situated on the secondary axis C the front lens L. The point A' vis the virtual position of the operators eye forthe rear lens and is used according to the rules given above to find the eccentricity or center C and the focal len h of lens L', 110 l just ,as A was used to find t e eccentricity or center C and the focal length of the front` lens L. The point C so determined generally falls very close to o'r even on the secondary axis Cs A of the front lens L so that the main axis C C of the com und lens produced passes in or close by t e opv erators e e. A

The inc 'nation ofthe front lens L being determined as just explained the prism 503 cemented thereon 'should preferably have its cemented face ground so that its reflecting face shall be inclined to properly reflect the central ray a b c a d coming along a' b vfrom the pivoted mirror' corresponding to 125 mirror 107, Fig. y1; for, when this is not done, the lower mirror must be made larger, that is, rather larger, as seen in Fig. 1, to make sure that it "will lill the prismatic mirror. Itl should be noted, however, that rule, therefore, I prefer to use a common 45 degrees prism for prism 503, and to set the front lens` so it will hold the reflecting,"

face of the 'prism inthebest position withrespect tothe pivoted mirror, as set forth in my said prior Case C, and to apply thecr rections for distortion. solely to l,the rear lens. In this way I am enabled to show the greatest possible'extent of the second image consistent with 4compactness, by using the largest possible prism, and the prism used 1s.

ofa standard type. Thus are secured at `the l same time efficiency, `compactness and facility ofl manufacture.

' The rear lens La can be very loosely mounted, even to the extent of having a certain play 0n its hinge 540, but the front lens L", when it carries the eye mirror as here shown, should be firmly connected with the frame and may be conveniently fixed thereto by means of plaster of Paris.

The separable form of eccentric' lens finder may be used without the mirrors purely as a field finder and is also useful even in combination with cameras that have not a separable compound objective. In such cameras it still .presents the 'advantages of forming a very perfect finder image and of having its inner lens faces easily vaccessible for cleaning. v c

The doublet of Fig. 8 has the proportions required to divide the work of refraction equally between the two lenses. In the doublet of Fig. 8, as in the single lens of Fig. 1, the eccentricity is variable within wide limits because the direction of the line of sight is fixed by sights or'range points located in the path of the incidental part of the central finder ray. Exact half lenses, therefore, may be used in Fig. '8, as in Figui, and for the same reasons, provided the two half lenses used have the different focal lengths required to secure finder fields having the two'desired different angular' values.

Where the doublet is to have but one field value, and is made separable, simply to a facilitate cleaning, the two half lenses used may have equal focal lengths as in my said Case A-p, where I make, as before stated, all claims on the half lens form of finder.

. NO'rE 1.-'Ilhe principle that minimum deviation favors orthoscopy is one of the oldest in the art. It was considered common' knowledge,.as early as 1827, by Airy, who is that the principleiof minimum deviation is not,` and cannot be applied generally for, in many cases its use is inconsistent with other essential requirements. y fr i NOTE 3.-I believe that I am the first'to f make a clear disclosure of what these essential requirements. must'be in any camera centric lenses, moreover, include heavy and heretofore 'unused marginal parts of the lens whose use involves unusual conditions, and these demanded original investigation, -which eventually led to my discovery ofLthe hitherto r overlooked but very important;l cocenter K.

NOTE 4.--The co-centeri-K isl a valuable contribution to lens theoryas well as to lens practice, in that it helps' to explain and render more scientificand precise, many well established, but not clearly understood empirical rules of lens construction.

NOTE v'5.-Herman, Gcometrical Optics Cambridge, 1900, page 196, gives a table withl full'Y data Ifor. the construction of a number of lenses,- either convergent or divergent 'according tothe sign of the focal length f, and having the certain radii of curvaturerho (p) and sigma (o) which insure-minimum spherical aberration for parallel rays received on the more highly curved or rho (p) face of the lens; but correction for spherical aberration in the image is not necessary, for reasons'given above inl co nection with Fig. 6, and the only defec to be considered is distortion. I

No'rE 6,-I-Ierschell describes a lens which may be Amade of such proportionsthat it shall be not only stigmatic but rigorously aplanatic for points such as A and Eof my p (See Fig. 53, Articles ,287 and 304 of Herschells treatise on Fig. 7, for instance.

.Light" in the E ncyclopacdia Mctropolitana, London, 1845, vol. 4, pages 341 to'586.'

distance AE; secondly, it would strictly satisfy the sine condition of' Abbe which, for angles as large as-those used in a finder field, is so inconsistent with the orthoscopic or tangent condition of Airy as to cause, in the present case, a very high degree of barrel distortion; thirdly, it would not utilize the minimum rays for its cci-center K would be situated to the left of the virtual eye at A instead of to the right of it. ,K

NOTE 8.-My first illustrated disclos '}1re'of v an optical finder element, mechanically gulded for convenient removal by a single motion of the hand, was made July 8, 1901, inoriginal Fig. 2 of my said' Case F, sub- JFi A'is

ner except by unmounting it, and this, morey over,neoess1tates the preliminary and temporary vunmounting of the front lens.

,No'rn 10.-My said Case C, based on the said French patent, referring to `the removable finder element (page 5, line 70) uses the term removed, for two diierent reasons: first, because the term removed (ac cording to Websters International, 1890) 1s g1- neric as to all methods of removal rea contemplated in such Case C, and, secondly, because it is more exactly descriptive of the special method of removal that ca n alone be used in the structure shown in sa1d Case C.

NOTE 11.-The finder lenses seen in Fig. 3 of my said Case C are both slightly inclined;

, but schinclinations are comparatively small and are accidentally present as incidental to other features, so that the axial line which passes through the thinnest point o`f the front lens and the thinnestpoint of the rear lens, is horizontal, insteadiof being inclined proved by inclining it as herein set forth;

hence the invention, in this articular, is not 4 de dent upon the exact ape of the lens.

at I claim as my invention, and desire to secure by Letters Patent, is: l,

1. The combination with a photographic camera, of a finder mounted-,thereon consist- 4 ing of an eccentric lens set with its principal axis inclined with respect to theprincipal axis of the camera.

2. The combination with a camera, of a compound eccentric lens finder, such lens 5 having-the line joining the optical centers of its elements inclined with respect to the princ'ilpal axis of thevcamera.

3. he combination with a camera of a compound eccentric lens finder therefor 5 mounted on the camera so as to admit finder rays distinct from the camera rays that are directed to enter the camera objective, said finder having one ofA its elements hinged to permit of turning said element out of the 0 path ofthe rays through the finder or back rinto said path.

I'n testimony whereof, I have signed my name to this specification.

JOSEPHv BECKER. 

